In a PFBC plant, normally a combustion chamber, containing the fluidized bed where fuel is burnt to generate combustion gases, and a cleaning plant, comprising a number of parallel groups of series-connected cyclones for the cleaning of the combustion gases, are placed within a common pressure vessel containing compressed combustion air. The flow resistance to gas passing through the combustion chamber and the cyclones gives rise to a fall in pressure so that the pressure existing in the cyclones is lower than that existing in the surrounding pressure vessel.
In the lowermost conical part of each cyclone and in the upper part of an outlet tube for solid separated material connected to the conical part, the gas mass and the solid material rotate at high velocity, which may lead to holes wearing in the conical wall or the tube wall. In the case of a hole in either wall, air from the surrounding space, where the pressure is higher than in the cyclone, will flow into the cyclone. This leads to a loss of combustion air, as well as a cooling of the combustion gases which are being used to drive a turbine. This means an energy loss. The air flowing in through such an erosion hole will also cause ignition of any unburnt fuel included in the separated material. Particularly in the case where the fuel is coal with a low ash content, the content of unburnt fuel in the separated material may be high and an intense combustion may result at the erosion hole. This combustion may lead to the initial size of the hole rapidly becoming greater or to a melt down of a substantial part of the outlet tube or the lowermost part of the cyclone.
The invention relates to a modified design of cyclone in which erosion damage in the lower part of the cyclone or in a tube used for discharge of cyclone-separated material can be indicated. The invention also relates to PFBC plant which can be safely operated for a considerable period of time after detectable erosion damage has occurred.